In previously known calorimetric measuring apparatus functioning according to the adiabatic principle, the decomposition vessel is placed into a boiler with water. The temperature increase of the water, approximately 3.degree. C., is measured, and is a measure of the calorific value. The measuring chamber, consisting of the decomposition vessel and the water boiler, is outwardly shielded by a water jacket that surrounds this measuring chamber on all sides; the temperature of this water jacket being adjusted to the measuring chamber temperature by heating and cooling. The high precision of this class of apparatus is achieved by great technical effort; nevertheless, penetration of the ambient temperature cannot be entirely prevented. Another drawback is the measuring time required, which can scarcely be reduced below 20 minutes per measurement.
In contrast, measuring apparatus according to the isothermal principle are distinguished by the fact that the boiler, which is filled with water and in which the decomposition vessel stands, is surrounded by a thermally stable jacket intended to prevent the transmission of energy from the outside to the inside. The energy released by the water-filled boiler to the thermally stable jacket cannot be detected. In the case of test specimens with the same energy content but with different combustion periods, this leads to different measurement results. The measuring time of these systems is no shorter than with the adiabatic principle.
The class of apparatus functioning according to the isoperibolic principle is also designed similarly to that of the isothermal measuring apparatuses. However, the decomposition vessel does not stand in a boiler, but is directly surrounded by a high-grade thermal insulator. The temperature increase of the decomposition vessel is measured by a temperature sensor and lies appreciably above the temperature increase observed with the adiabatic measurement principle or with the isothermal measurement principle. The fact that the decomposition vessel heats up considerably leads to a transfer of energy to the insulating jacket, which can neither be measured nor influenced. A heat front develops on the interior wall of the insulating vessel facing the decomposition vessel, and migrates into the insulation. The heat fronts developed during subsequent measurements overlap each other and, depending on the temperature level, energy from earlier measurements is reflected on the decomposition vessel. Distortions of the measured values are caused in this way. Devices designed according to this principle allow short measurement periods, but with limited accuracy.